Current transformers with improved coaxial feed

ABSTRACT

Current transformer apparatus includes a toroidal core with a secondary winding thereabout. A metallic shield and coupling member surrounds the core and has a central longitudinal opening therein for the insertion of a coaxial current carrying conductor, the current of which is to be sensed. The shield and coupling member within the longitudinal opening is laterally slotted about the cylindrical surface thereof to allow direct and close electromagnetic coupling between the coaxial conductor and the core. In one embodiment, an inner shield is provided about the core and is slotted about the exterior periphery thereof. The impedance of the coaxial conductor and the transformer may be matched by a coaxial line extending through the opening in the shield and coupling member to optimize coupling. In another embodiment, a secondary winding terminating resistance is variable by inclusion in a removable coaxial resistance cartridge. Several transformers may be cascaded in tandem by passing the output of one coaxially into the input of another.

United States Patent Anderson 1 Oct. 24, 1972 John M. Anderson, Scotia, NY.

[73] Assignee: General Electric Company [22] Filed: Dec. 14, 1970 [2]] Appl. No.: 97,918

[72] Inventor:

[52] US. Cl. 323/6, 324/127. 336/175 [51] Int. Cl. ..G0lr 19/00, HOlf 40/06 [58] Field of Search .....32l/9 R; 323/6, 44 R, 61, 74;

Primary Examiner-A. D. Pellinen Attorney.lohn F. Ahem, Paul A. Frank, Richard R. Brainard, Jerome C. Suuillaro, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman [57 ABSTRACT Current transformer apparatus includes a toroidal core with a secondary winding thereabout. A metallic shield and coupling member surrounds the core and has a central longitudinal opening therein for the insertion of a coaxial current carrying conductor. the current of which is to be sensed. The shield and coupling member within the longitudinal opening is laterally slotted about the cylindrical surface thereof to allow direct and close electromagnetic coupling between the coaxial conductor and the core. in one embodiment, an inner shield is provided about the core and is slotted about the exterior periphery thereof. The impedance of the coaxial conductor and the transformer may be matched by a coaxial line extending through the opening in the shield and coupling member to optimize coupling. in another embodiment, a secondary winding terminating resistance is variable by inclusion in a removable coaxial resistance cartridge. Several transformers may be cascaded in tandem by passing the output of one coaxially into the input of another.

9 Claims, 7 Drawing Figures PATENTEDUIIT 24 I972 INVENTOR Jo M. AND SON fly HIS ATTORNEY CURRENT TRANSFORMERS WITH IMPROVED COAXIAL FEED This invention relates to improved high frequency transformers. More particularly, the invention relates to such transformers having structural features which optimize electromagnetic coupling and shielding and which provide coaxial coupling and facilitate the optimization of load resistance characteristics. This invention is related to my co-pending, concurrently filed application, Ser. No. 97.919, now US. Pat. No. 3,629,693, which is assigned to the assignee of this invention.

As is set forth in greater detail in the aforementioned co-pending application, the specification of which is incorporated herein by reference thereto, the attainment of undistorted high frequency output from current transformers is a challenge to one skilled in the art and includes the optimization of several seemingly inconsistent parameters. In accord with the electrical characteristics of current transformers constructed utilizing the techniques of my aforementioned invention, such undistorted high frequency operation is achieved largely by providing oscillation-damping resistances periodically along the secondary winding thereof, and by utilizing terminating resistances which improve low frequency response in order to optimize the figure of merit of the transformer without unduly sacrificing high frequency performance. Even utilizing the teachings of my aforementioned invention, however, the construction of transformer apparatus for the attainment of undistorted high frequency response nevertheless requires the solution of difficult electromechanical problems.

One such problem which must be overcome is the presence of electrostatic fields which, if coupled into a high frequency current transformer, can preclude undistorted operation. Similarly, spurious electromagnetic fields, from remote portions of the current-carrying conductor which constitutes the primary of the current transformer, must be avoided. Although shielding is effective to eliminate unwanted fields, it is often not consistent with the necessity of effective coupling between the primary conductor, on one hand, and the secondary and core on the other hand.

Another problem attendant the use of shielding in a current transformer is that shields, at high frequencies, tend to approximate microwave cavities and tend to resonate, introducing high frequency distortion.

Still another problem in the construction of current transformers is the seeming inconsistency between the requirements for freedom from frequency distortion and the attainment of high sensitivity on one hand, both of which are favored by the use of a relatively high resistance terminating the secondary winding and, on the other hand, the desirability for a reasonably high current-time figure of merit, which is favored by a low terminating resistance. Another advantage of a low terminating resistance is the consequent reflection of low resistance into the primary conductor and, consequently, low resistive losses and improved coupling. Another aspect of this problem relates to the desirability for flexibility in being able to change secondary terminating resistances for any given transformer so that performance for any particular usage may be optimized without the necessity of going to a different transformer.

Accordingly, it is an object of the invention to provide high frequency current transformers which overcome one or more of the foregoing disadvantages and overcome the foregoing problems.

Another object of the invention is to provide current transformer apparatus for achieving effective electromagnetic coupling, while at the same time avoiding the effects of undesirable fields, both electrostatic and electromagnetic.

Still another object of the invention is to provide current transformer apparatus which provides the advantages of high and low terminating resistance and flexibility in optimizing the same.

Yet another object of the invention is to provide current transformer apparatus which exhibits high sensitivity and yet reflects a very low resistance into the primary conductor.

Briefly stated, in one accord with one embodiment of the invention, I provide current transformer apparatus including a toroidal core adapted to surround a current carrying conductor and a secondary winding thereabout. The core and secondary are surrounded by a metallic shield and coupling member having a hollow cylindrical central section which is adapted to receive a coaxially disposed current can'ying conductor. The surface of the cylinder central section thereof is azimuthally slotted to provide means for direct and effective coupling between primary and secondary, but yet provides adequate shielding from spurious electromagnetic and electrostatic fields. In accord with another feature of the invention, the potential resonance of the shield is damped by resistances placed across the slot. In accord with still another feature of the invention, means are provided for removably inserting any preselected terminating resistance across the secondary of the transformer. In still another embodiment, any desired number of current transformers are ganged in tandem by enclosing the output means of one transformer concentrically within the input means of another transformer.

The novel features believed characteristic of the present invention are set forth in the appended claims.

The invention itself, together with further objects and advantages thereof may best be understood by reference to the following detailed description, taken in connection with the attached drawing, in which:

FIG. 1 is an electrical schematic of a current transformer constructed in accord with the invention,

H0. 2 is a sectioned perspective view of a transformer in accord with the invention illustrating the shield structure thereof,

H08. 3 and 4 show alternative shield structures, illustrating alternative embodiments of the invention,

P10. 5 is a partially sectioned, partial vertical crosssectional view of an impedance matching feature useful in transformers constructed in accord with the invention,

FIG. 6 is a perspective view of a removable terminating resistance cartridge in accord with another feature of the invention, and

FIG. 7 illustrates two current transformers ganged in tandem in accord with yet another feature of the invention.

FIG. I illustrates atypical electrical circuit for a current transformer in accord with the invention. In FIG.

I, transformer includes a core 11, which may be a series of ferromagnetic laminations for low frequency usage, a ferrite core for intermediate and high frequency usage. or an air core for a very high frequency range of operation. A secondary winding 12 includes a plurality of turns 13 which encircle core 11 and which terminate in a pair of secondary winding leads 14 and 15. Winding I2 is wound about the core 11 and is electrically terminated in a characteristic resistance 16 which is chosen to have a relatively low value as compared with such resistances of the prior art, in order to optimize high frequency response of the transformer and to cause reflection of a minimum of impedance into the primary conductor.

Transformer 10 also includes, in accord with the invention. a plurality of oscillation-damping resistances 18 which are connected in electrical circuit between periodically occurring turns 13 of secondary winding 12 and a common electrical conductor 19 which is preferably a low inductive conductor, as for example, a flat circular washer 20, which is preferably not electrically connected to any other portion of the transformer, but is allowed to float.

FIG. 2 is a vertically sectioned perspective view of a transformer which physically embodies the electrical structure described with respect to FIG. 1 and which illustrates one expletive embodiment of coupling and shielding means in accord with the invention. As illustrated in FIG. 2, transformer 10 includes core 11 with winding 12 thereabout and a plurality of oscillationdamping resistances 18 connected between individual turns of winding 12 and a common electrical conductor. The core and secondary are enclosed in a metallic shield and coupling means 21 which, in this embodiment, has the configuration of a closed metallic rhombahedral enclosure having a central cylindrical opening 22 therein with a longitudinal sidewall 23 having an azimuthal slot 24 therein which slot is symmetrically located with respect to core 11. As is illustrated schematically in FIG. 1, a primary conductor 17 passes through core 11. In FIG. 2, such a conductor passes concentrically within central opening 22 in shield and coupling means 21 so as to couple, coaxially, through slot 24 into core 11 with a minimum of loss. Shield and coupling means 21, with the central aperture 22 and slot 24 in wall 23 thereof, provides an ideal balance between the seemingly divergent objectives of electrostatic and electromagnetic shielding while permitting highly effective coupling between primary and secondary. In FIG. 1, secondary 12 is illustrated as terminating in a resistance 16 and having output conductors 14 and 15. In FIG. 2 a coaxial conductive cartridge 25 protrudes from shield and coupling means 21 and includes conductors 14 and 15.

FIG. 2 illustrates the basic structure for the coupling and shielding means in accord with the present invention. FIGS. 3 and 4 illustrate alternative and improved modifications thereof. In FIG. 3, primary conductor 17 is represented as a coaxial line having an exterior condoctor 26 and an interior conductor 27. Exterior conductor 26 is electrically connected to shield and coupling means 21 and interior conductor 27 passes coaxially through opening 22 in means 21 and energy is coupled therefrom through aperture 24 into core 11. In accord with this embodiment of the invention, spurious oscillations of the cavity created within coupling and shielding means 21 is prevented by oscillation-damping resistances 28 which are interposed, periodically around the azimuthal slot 24, across the same. Alternatively. resistances 28 may be a continuous film resistance having typically a resistance value of 50 ohms/square. As is mentioned hereinbefore, at very high frequencies, the cavity within shield and coupling means 21 tends to approximate a microwave cavity and has a characteristic frequency at which resonance is inherent and oscillations occur. In accord with the present invention, such resonances are obviated by damping resistances 28, which effectively lower the Q of the coaxial cavity and thereby prevent any oscillation at high frequencies.

In FIG. 4, an alternative structure for coupling and shielding means 21 is illustrated schematically, showing a similar coaxial feed for primary conductor 17. In FIG. 4, coaxial primary conductor 17, including exterior conductor 26 and interior conductor 27, passes coaxially through cylindrical opening 22 in the center of shield and coupling means 21 with exterior conductor 26 electrically in contact with the shield and coupling means 21 and the interior conductor passing coaxially through opening 22 so as to couple through cylindrical slot 24 and into core 11. Within shield and coupling means 21 is a second toroidal shield member 29 which is closely disposed about core member 11 and closely spaced to, but electrically isolated from the exterior shield and coupling means 21 by a sufficiently small space as to minimize capacitances, and having an exterior peripheral slot 30 therein. The provision of slots 24 and 30 at opposite radial sides of toroidal core 11 provides essentially perfect shielding therefor from all spurious electromagnetic and electrostatic fields that are undesired, while at the same time providing effective means for energy transmitted into shield and coupling means 21 to be effectively coupled to core 11 without any substantial losses. Resistances 28 illustrated in FIG. 3 may be utilized in this structure, if desired.

As is illustrated in FIGS. 3 and 4, in accord with the present invention, the primary conductor is coaxially coupled within the opening in the coupling and shielding means in transformers in accord with the invention. For optimum performance, it is desirable that this coupling be performed with a maximum of efficiency. Maximum coupling efficiency may be achieved by matching the impedance of the primary conductor to the coupling and shield means impedance. In accord with the present invention, I achieve this by providing matching means 31 centrally located within the aperture or opening within shield and coupling means 21. Such impedance matching means is shown schematically in FIG. 5 of the drawing.

In FIG. 5, shield and coupling means 21 having opening 22 therein is further modified so that impedance matching means 31 is inserted within opening 22 and coaxial conductor 17 is connected to respective ends of matching means 31. Matching means include exterior conductor 32 which is electrically connected to shield and coupling means 21 and an interior conductor 33 which is electrically connected to the inner conductor 27 of primary conductor 17 as for example, by coupling 34. Impedance matching is accomplished by impedance means 35 which is operatively associated with inner conductor 33 and is represented schematically by a dotted line box in FIG. 5. While specific impedance means for modifying the impedance of coaxial conductor 17 to match the characteristic impedance of the shield and coupling means 21 and core 11 are not shown, many such impedance means are well-known to the art and will readily occur to the skilled worker.

As is mentioned hereinbefore, it is highly advantageous for the users of current transformers to be able to preselect a predetermined value of secondary winding terminating resistance. The reasons for this are set forth in greater detail in my aforementioned c0 pending, concurrently-filed application and hereinbefore, but will be reviewed briefly. Thus, while secondary terminating resistances of approximately 0.] ohm to 50 ohms may be utilized in accord with the transformers of the invention, it has been found that, within that range, low load resistances are consistent with improved high frequency response and similarly cause the reflection of a minimum impedance into the primary conductor, thereby reducing resistive losses and facilitating impedance matching. On the other hand, high terminating resistances are consistent with the achievement of high sensitivity, that is, a high change in output voltage per ampere of current sensed. For any given usage, it is desirable that the specific resistance be chosen which is optimum from both points of view for the particular use. It is very inconvenient and expensive to fabricate a special transformer for each use or substitute one transformer in an apparatus for another, merely to change determining load resistance. Accordingly, in accord with the present invention, I provide preselectable terminating load resistances, or burdens as they are known in the art, in the form of coaxial cartridges which may be removably inserted in the electrical output circuit of the current transformer, as for example, by screw threads to cause such a cartridge to be readily removable and a different cartridge substituted in order to achieve the desired terminating resistance value.

As is illustrated in FIG. 6 of the drawing, cartridge 25 includes threads 36 and knurled grips 37 so that a different terminating resistance may be inserted into the electrical circuit between the primary winding 12 and the coaxial output terminals 14 and 15. Although at low frequencies, conventional lump parameter resistances may be utilized in order to provide the desired terminating resistance, for frequencies in excess of megahertz it is advantageous to use thin film coaxial resistances in order to provide optimum frequency response and provide the desired and necessary resistance to the circuit. In my aforementioned co-pending application, there is disclosed secondary winding means for assuring a rapid rise-time characteristic to insure that current waveform in the secondary winding faithfully follows the primary current waveform. In further accord with this invention, I provide means for assuring this accurate following over a wide frequency range. Thus, rather than placing only differing terminating resistances in cartridge 25, I cause the cartridge to contain alternative total terminating circuits, which may be simple or complex, as is well-known to those skilled in the art, for providing reactive compensation to maintain a proper rise time characteristic over a wide range of frequencies. One such circuit means may comprise a resistor and an inductor in series across the secondary winding.

In further accord with the invention, I provide a solution to the problem in instances in which a low impedance reflected into the primary conductor is desired and, at the same time a good high frequency response and a good figure of merit are desired. Ordinarily it is exceedingly difi'icult to achieve such desired characteristics with a single transformer, even utilizing the inventive concept set hereinbefore. In solution to this problem in accord with the invention, a plurality of individual current transformers, each having the appropriate characteristics, are ganged or coupled in tandem for readily achieving the seemingly-impossible design criteria. As is apparent from an examination of FIG. 2, the input means of the current transformer 10 is readily adapted to receive the output means of a similar current transformer having different dimension. Such a combination is illustrated in FIG. 7. More specifically to feed the output of one transformer into the input of the other, the output is made coaxial with the outer conductor grounded to the shield and coupling means 21 of the first transformer and the center conductor 27 passing coaxially through the aperture 41 in the shield and coupling means of the second transformer, as is illustrated in FIG. 3 in detail. As a modification of FIG. 3, the center conductor, once passing through the shield and coupling means, contacts a terminating end which electrically contacts the outer end of the apertured central cylinder.

In FIG. 7, a first current transformer 10 having a shield and coupling means 21 with a central longitudinal aperture 22 therein and a coaxial output means 25, may be coupled coaxially into a second, similarlydesigned transformer having analogous construction features but being substantially smaller, so that the central aperture 41 thereof just fits over output means 25 of transformer 10 and which includes its own output means 42. End cap 43 of output means is connected to the central conductor thereof and terminates in electrical contact with the outer side of shield and coupling means 44 of transformer 40. This combination readily provides a composite characteristic different from the characteristics of the individual transformers. With such a combination, I am able by this feature of the invention to extend the range of currents measurable without sacrificing the frequency range of the transformers of the invention. Thus, for example, this embodiment readily provides current sensing means operative at frequencies in excess of I00 megahertz which reflect impedances only as low as 10" ohms into the primary conductor. This may be accomplished, for example, by choosing as transformer 10, one with good figure of merit and low terminating impedance characteristics and by choosing as transformer 40 one with high sensitivity, and a higher terminating resistance characteristic, thereby to achieve a combined set of desirable operating characteristics which may not be achieved with a single transformer.

Current transformer 10 may have, for example, a toroidal core of common laminated silicon iron transformer material with a cross-section 2 inches on a side and an CD. of 12 inches and an ID. of 8 inches. One hundred turns of No. 3 circular cross-section copper wire are wound around the core and exhibit a DC resistance of 0.017 ohms and an inductance of 0.45 henry. A lower cut-off frequency for this transformer is approximately 6 X 10' Hertz. A terminating resistance of in the form of a coaxial shunt or the equivalent thereof is provided, resulting in an overall sensitivity or transfer impedance of approximately 1 microvolt per primary ampere. A figure of merit of 1,850 ampere seconds is realized from this transformer alone and a peak magnitude of primary current of 350,000 amperes at 60 Hertz may be measured without saturation. Because of this high current and high power operation, it may be necessary to water cool this transformer. Such water cooling is well-known to the art and does not provide an impediment. Over the modified coaxial output 25 of transformer 10, there is coupled a second transformer 40 having an aperture 41 which just fits over output means 25 of transformer 10. Transformer 40 may conveniently utilize a ferrite core of 06 Ferrite Material available from Indiana General, having an CD. of approximately 3.5 inches, an ID. of approximately 2.0 inches and a thickness of approximately 0.5 inches. One hundred turns of No. 16 insulated magnet wire are used as the secondary. The DC resistance of the secondary is approximately 0.095 ohms and the inductance is approximately 41 millihenries. Twenty-five damping resistances of 1,000 ohms each are spaced one every four turns around the secondary to damp coil resonance. The secondary is terminated in a one ohm resistance providing a reflected resistance into the primary, which in this instance is the output means 25 of transformer 10, of approximately l0" ohms. A figure of merit of approximately ampere seconds is achieved at a continuous 2 watt input of 400 amperes. This transformer is operative to operate very effectively to a range of in excess of 100 megahertz. Although the low frequency response is not good below 0.5 Hertz, if it is desired to maintain the low frequency of the larger iron core transformer, such may readily be achieved by the utilization in transformer 40 of an iron core having similar characteristics to the ferrite core thereof.

In addition to the combination of transformers and 40 is illustrated in FIG. 7, further transfonners may be coupled in tandem or cascaded by the progressive insertion of a modified output means 42 of transformer 40 into a coaxial aperture in yet another, higherfrequency, more sensitive, higher reflected impedance transformer.

By the foregoing, I have disclosed improved circuit transformer apparatus which achieves unique shielding from spurious electromagnetic and electrostatic fields and allows for high frequency operation with a maximum of effectiveness, particularly in coupling which is achieved by coaxial feed from a primary conductor to the transformer. Means are provided for damping spurious oscillations of the shielding means. Means are further provided for impedance matching between the primary conductor and the current transformer for maximum power transfer and coupling efficiency. Means are still further provided to utilize transformers over a wide range of applications by removable burdens or load resistances and load impedances to optimize rise-time at a wide range of frequencies. Other means are provided for coupling a plurality of current transformers in cascade or tandem in order to achieve a very effective balance between seemingly inconsistent, but desirable parameters. While the invention has been set forth herein with respect to certain specific embodiments and examples thereof, many modifications and changes will readily occur to those skilled in the art. Accordingly, by the appended claims, I intend to cover all such modifications and changes as fall within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. Current transformer apparatus comprising: a toroidal core adapted to surround a conductor, the current of which is to be sensed; a secondary winding about said core; coaxial means for electromagnetically coupling the current carrying conductor with said core and for shielding said core from spurious fields, said coupling and shielding means comprising a metallic shield enclosing said core and said winding and having a hollow cylindrical central section including an aperture for passing the conductor therethrough, said central section including a cylindrical wall which is slotted for effective electromagnetic coupling between said conductor and said core; means terminating said secondary winding with a predetermined resistance; and, means for extracting an electrical signal from said secondary winding.

2. The transformer apparatus of claim 1 wherein said core is further surrounded by an inner shield which is slotted about the outer periphery thereof.

3. The transformer apparatus of claim 1 wherein the slotted shield is provided with oscillation-damping resistances to prevent resonance thereof within the operating frequency range of the transfonner apparatus.

4. The transformer apparatus of claim 1 wherein the resistance terminating said secondary winding is contained in a removable cartridge mat is mechanically integral with said means for extracting the electrical signal.

5. The transformer apparatus of claim 1 further comprising second current transformer apparatus of like structure to the first current transformer apparatus defined in claim 12, said second current transformer apparatus being serially connected with the first current transformer apparatus by fitting the aperture of the central section of the second current transformer apparatus over the signal extraction means of the first current transfonner apparatus and establishing electrical contact between the metallic shields of the first and second current transformer apparatus.

6. The transformer apparatus of claim 4 wherein said terminating resistance is within the range of approximately 0. l to 50 ohms.

7. The transformer apparatus of claim 6 wherein said terminating resistance is in the form of a thin film coaxial resistor.

8. Transformer apparatus of claim 1 wherein said terminating resistance is preselectable to any desired value by the location thereof in a removable, replacable cartridge within said shield.

9. The apparatus of claim 8 wherein said removable cartridge contains a complete terminating circuit including reactive elements effective to provide means for causing the rise time characteristic of said transformer apparatus to be acceptable over a wide range of operating frequencies. 

1. Current transformer apparatus comprising: a toroidal core adapted to surround a conductor, the current of which is to be sensed; a secondary winding about said core; coaxial means for electromagnetically coupling the current carrying conductor with said core and for shielding said core from spurious fields, said coupling and shielding means comprising a metallic shield enclosing said core and said winding and having a hollow cylindrical central section including an aperture for passing the conductor therethrough, said central section including a cylindrical wall which is slotted for effective electromagnetic coupling between said conductor and said core; means terminating said secondary winding with a predetermined resistance; and, means for extracting an electrical signal from said secondary winding.
 2. The transformer apparatus of claim 1 wherein said core is further surrounded by an inner shield which is slotted about the outer periphery thereof.
 3. The transformer apparatus of claim 1 wherein the slotted shield is provided with oscillation-damping resistances to prevent resonance thereof within the operating frequency range of the transformer apparatus.
 4. The transformer apparatus of claim 1 wherein the resistance terminating said secondary winding is contained in a removable cartridge that is mechanically integral with said means for extracting the electrical signal.
 5. The transformer apparatus of claim 1 further comprising second current transformer apparatus of like structure to the first current transformer apparatus defined in claim 12, said second current transformer apparatus being serially connected with the first current transformer apparatus by fitting the aperture of the central section of the second current transformer apparatus over the signal extraction means of the first current transformer apparatus and Establishing electrical contact between the metallic shields of the first and second current transformer apparatus.
 6. The transformer apparatus of claim 4 wherein said terminating resistance is within the range of approximately 0.1 to 50 ohms.
 7. The transformer apparatus of claim 6 wherein said terminating resistance is in the form of a thin film coaxial resistor.
 8. Transformer apparatus of claim 1 wherein said terminating resistance is preselectable to any desired value by the location thereof in a removable, replacable cartridge within said shield.
 9. The apparatus of claim 8 wherein said removable cartridge contains a complete terminating circuit including reactive elements effective to provide means for causing the rise time characteristic of said transformer apparatus to be acceptable over a wide range of operating frequencies. 